Key Physics and Doppler Principles

Size: px

Start display at page:

Download "Key Physics and Doppler Principles"

Norma Tucker
5 years ago
Views:

1 Key Physics and Doppler Principles Robert A. Levine, MD, FACE, ECNU Thyroid Center of New Hampshire Geisel School of Medicine at Dartmouth College AACE/ACE Advanced Neck Ultrasound Training Course

2 Disclosures: No relevant financial or corporate conflicts of interest. The use of investigational drugs will not be discussed

3 Image Optimization Create the sharpest image to allow tissue discrimination. Equipment factors: Quality of Transducer Quality of Electronics Image Enhancement and Compound Imaging User Adjustments: Depth, Gain, Frequency Focal zones Number and Location Compound Imaging Tissue Harmonic Imaging Dynamic range Doppler settings: Wall filter and PRF

5 Optimal Depth

6 Image Optimization Gain Overall Gain Time Gain Compensation Multiple channels corresponding to depth User adjustable to achieve best image quality at region of interest Focal Zone(s) Adjustable depth and number Greater number of zones slows refresh rate

7 Optimal Gain

8 Compare echotexture of thyroid parenchyma to strap muscles and SCM.

9 Optimal Time Gain Compensation

10 Resolution Resolution is the ability to discriminate two structures as separate entities Types of resolution : Axial (distance from the transducer) Lateral (transverse) Azimuthal (thickness of imaging plane)

11 Resolution AZIMUTHAL (up-down) AXIAL (distance from transducer) LATERAL (side to side)

12 Resolution Ability to discriminate two adjacent objects as separate entities. The narrower the beam, the better the lateral resolution. The higher the frequency the better the axial resolution. Poor focus Image Good focus

Focus and Resolution Focused beam width determines Lateral and Azimuthal Resolution Near field (Fresnel Zone) Large variations of intensity Far field (Fraunhofer Zone) Greater variation with greater

13 Focus and Resolution Focused beam width determines Lateral and Azimuthal Resolution Near field (Fresnel Zone) Large variations of intensity Far field (Fraunhofer Zone) Greater variation with greater distance. Focal Zone - Area of maximal narrowing Pulse duration (frequency) determines Axial Resolution (axial resolution = 1/2 spatial pulse length) Practical Consideration - As frequency increases, axial resolution improves, but depth of imaging decreases. The number and depth of the focal zones are often adjustable and indicated on the display

14 Adjustment of number and position of focal zones

15 Adjustment of number and position of focal zones

16 Single focal zone

17 Four focal zones

18 Frequency and Resolution Higher frequency gives better resolution. Higher frequency gives less penetration. Need to find best compromise for depth of interest

19 Image Optimization - Frequency Choose highest frequency (12-15 MHz) that allows adequate depth penetration. Lower frequencies (7-10 MHz) for deep structures or very obese subjects

20 Frequency and depth

21 Which of the following is true regarding ultrasound frequency? 1. Increasing frequency will increase depth of penetration. 2. Increasing frequency will improve resolution. 3. Increasing frequency will increase noise. 4. Both 1 and 2 are correct.

22 Advances in Technology Signal Processing Image Enhancement Noise reduction Edge sharpening Utilization of CT and MRI reconstruction algorithms Beam Steering Spatial compounding

23 Image Optimization- Compounding

24 Signal Processing Compunding

25 Comparison of standard and processed images

26 Effect of Compound Imaging on Artifacts Comet Tails

27 Effect of Compound imaging on Artifacts Edge Artifact and Enhancement

28 Effect of Compound Imaging on Spongiform Nodule

29 Image Optimization Dynamic range May increase conspicuity of subtle lesions

30 Doppler Shift

31 Color and Power Doppler Meritt, 1998

32 Color and Power Doppler Meritt, 1998

33 Color and Power Doppler Color Doppler Provides information regarding direction and velocity. More useful in vascular studies Power Doppler No information regarding velocity Less angle dependence Less noise Increased sensitivity for detection of flow

In malignant nodes aberrant vessels enter peripherally in the

34 Doppler Lymph nodes In normal nodes vessels enter centrally at the hilus, and spread along the long axis. In malignant nodes aberrant vessels enter peripherally in the node capsule. Increased (disordered) vascularity may be seen peripherally and centrally.

35 Doppler of Nodes Demonstration of Chaotic or peripheral vascularity in malignant nodes Can be seen in reactive nodes Normal vascularity is reassuring Power Doppler for high sensitivity Use low wall filter Use a low PRF < 800 Low wall filter and low PRF both increase the sensitivity for detection of low flow.

36 Achieving the highest sensitivity with Doppler imaging. High Doppler sensitivity needed for lymph nodes. Power Doppler Maximum Doppler gain without noise. Low Pulse Repetition Frequency PRF < 800 Low wall filter.

37 PRF

38 PRF 700 Wall filter varies

40 Image Optimization Summary and Conclusions High quality equipment is preferable, BUT a great ultrasonographer using low quality equipment will obtain better images than a lousy ultrasonographer with great equipment. I need to find out what type of piano Mozart played so I can sound like him. User adjustments of gain, depth, frequency, focal zones, dynamic range, spatial compounding, pulse repetition frequency, and wall filter will give the optimal image quality.

Principles of Ultrasound Imaging Image Optimization

Principles of Ultrasound Imaging Image Optimization Robert A. Levine, MD, FACE, ECNU Thyroid Center of New Hampshire Geisel School of Medicine at Dartmouth College Disclosures: No relevant financial or